1. Field of the Invention
The present invention relates to a magnetic disk for use in a device such as magnetic disk cartridge, and more particularly to a magnetic disk having a disk-shaped magnetic sheet and a center hub.
2. Description of the Related Art
The magnetic disk cartridge mainly comprises a cartridge case or jacket constituted by upper and lower cases or jackets and having a head access aperture, the magnetic disk being accommodated rotatably in the cartridge case, and a shutter for opening/closing the head access aperture in the cartridge case.
The magnetic disk is constituted by a flexible disk-shaped magnetic sheet having an aperture in its central portion, and a center hub having a central protrusion with a flange at its outer periphery. The central protrusion of the center hub is inserted into the central aperture of the magnetic sheet, while the flange of the center hub is bounded or adhered to an inner peripheral portion of the magnetic sheet.
As is illustrated in FIG. 8, a center hub 6 is punched in the direction X from the flange 9 of the center hub 6 to from a projection part 8, which produces a burr 7 on the edge of the flange 9 in the same direction of the projection part 8.
FIG. 8 also shows that a surface 9a of the projection part 8 of the flange 9 becomes the adhered area with the magnetic sheet 5, shown by the imaginary line. In this case, if the burr 7 protrudes to the side of the flange surface 9a, the bonding of the flange 9 with the magnetic sheet is incomplete and, as a result, the magnetic sheet 5 and the center hub 6 may become detached during the usage of the magnetic disk.
As the protruding lengths of burr 7 are not uniform in their directions, if the magnetic sheet 5 and the center hub 6 are bonded together with the burr 7 protruded to the flange surface side 9a, corrugation may occur especially on the surface of inner peripheral area of the flexible magnetic sheet 5.
To overcome this shortcoming, as indicated by the arrow X in FIG. 9, it has been suggested to punch the center hub 6 from the side of the projecting part 8 through the flange 9 whereby the burr 7 is formed on the opposite side of the projection part 8. However, if the center hub 6 with the burr 7 protruding from the flange 9, as illustrated in FIG. 9 is used, the burr 7 will face the inner surface of the upper jacket 1 (FIG. 10). As is generally known in the field of the magnetic disk 4, a spindle 10 is inserted into a spindle hole 11 of the center hub 6 and, with the cooperation of a drive pin 12, the horizontal location of the magnetic disk is defined or established. The insertion of the drive pin 12, which rotates simultaneously with the spindle 10, into the drive pin hole 13, enables the magnetic disk 4 to rotate.
When a magnetic-disk cartridge is placed in a recording and reproducing unit, the drive pin hole 13 of the center hub is not necessarily located at a position of the drive pin 12 of the recording and reproducing unit but, as illustrated in FIG. 10, usually is in the position of its rotating locus. In this position, the end of the drive pin 12 revolves into sliding contact with the bottom surface of the center hub 6 while being biased by a compression spring (not illustrated), and when the drive pin 12 is rotated to a position facing to the drive pin hole 13, the drive pin 12 is inserted into the drive pin hole 13 by a spring-back force of the spring, whereby the magnetic disk 4 starts to rotate. However, until the drive pin 12 is located in the drive pin hole 13, the center hub 6 keeps slanting with one section of the hub 6 being pushed up by the spring loaded on the drive pin 12. If the center hub 6 slants, as described above, the inner side of the upper jacket 1, made of a synthetic resin, is scratched by the burr 7, thus producing shavings, scratched-out pieces or powder therefrom. The shavings may also be produced by a vibration which causes the tip of the burr 7 to scratch the inner part of the upper jacket 1 when the magnetic disk cartridge is being carried around, so that the shavings stick to the magnetic sheet 5, which produces an adverse effect to the recording and reproducing of signals.